1. FIELD OF THE INVENTION
The present invention relates generally to a beam index type color television receiver, and is directed more particularly to a beam index type color television receiver having a beam index type color cathode ray tube in which the scanning velocity of the cathode beam is modified.
2. Description of the Prior Art
A conventional beam index type color television receiver includes a beam index type color cathode ray tube in which an electron gun emits a single electron beam which scans across a phosphor screen consisting of red, green and blue vertical color phosphor stripes arranged to repeat at intervals, in the horizontal direction and index phosphor stripes arranged on the inner surface of the phosphor screen to repeat at intervals in the horizontal direction. In such a receiver, the color switching of the single electron beam is carried out independence upon an index signal which is produced when the electron beam scans the index phosphor stripes, so that the electron beam is density-modulated by a red primary color signal when the electron beam scans the red color phosphor stripes, by a green primary color signal when the electron beam scans the green color phosphor stripes, and by a blue primary color signal when the electron beam scans the blue color phosphor stripes.
According to conventional techniques, in order to increase the luminance and the degree of color saturation in the color cathode ray tube, it has been proposed that the scanning velocity of the electron beam should be modulated by a signal with a constant amplitude and also by a so-called write-in frequency which is determined by the repetition frequency, or periodic spacing of the color phosphor stripes and the scanning velocity of the electron beam.
For example, as shown in FIG. 1, an index phosphor stripe I is provided at positions among red, green and blue color phosphor stripes R, G and B at the ratio of three index phosphor stripes I for each two triplets or set of three of the color phoshpor stripes R, G and B. A signal SV with a constant amplitude and a frequency 3 f.sub.T, that is, three times as the write-in frequency, or triplet frequency f.sub.T, with the latter being determined by the repetition frequency of one set of the color phosphor stripes and the scanning velocity of the electron beam, is supplied to a horizontal deflection device for modulating the scanning speed at the phase shown in FIG. 1 to modulate the scanning velocity of the electron beam. As a result, the scanning velocity of the electron beam is so modulated that the beam is focussed or converged toward a point b from a point a as indicated by arrows in FIG. 1. In other words, the time period in which the electron beam scans the color phosphor stripes R, G and B becomes longer to increase the luminance and the degree of color saturation.
According to this method, however, the time period in which the electron beam scans the portion between adjacent color phosphor stripes R, G and B is shortened and accordingly the time period of scanning the index phosphor stripe I becomes short. Therefore, since the level of the index signal is determined by the product of the magnitude of the beam current and the time period in which the electron beam scans the index phosphor stripe I, the level of the index signal is lower than the level of the index signal in a receiver in which the scanning velocity of the electron beam is not modulated. Therefore, although the beam current is large when the picture displaced on the screen is bright, this method works adequately. However, when the picture displayed on the screen is dark and hence the beam current is small, the level of the index signal becomes relatively low and the signal-to-noise ratio of the index signal becomes so low that accurate color reproduction can not be carried out.
Also, when a so-called write-in signal, which corresponds to the signal selected from the respective primary color signals, is supplied through a video amplifier as a drive voltage E.sub.D (FIG. 1) to, for example, a first grid of the color cathode ray tube, the, a drive voltgage E.sub.D applied to the first grid tends to lack sharp rising and falling edges owing to the characteristic of the video amplifier. Here, as shown in FIG. 1 the drive voltage E.sub.D corresponds to the reproduction of, for example, green color only. Because of the lack of sharpness encountered in the video amplifier, the scanning positions of the electron beam on the color phosphor stripes are displaced and hence the luminance and the degree of color saturation are lowered. However, if the scanning velocity of the electron beam is modulated as set forth above, the lack of sharpness in the rising and falling edges of drive voltage E.sub.D due to the characteristic of the video amplifier is not a significant problem, and the luminance and saturation are significantly improved.
The lack of sharpness in the rising and falling edges of drive voltage E.sub.D due to the characteristic of the video amplifier is not uniform with the amplitude of drive voltage E.sub.D, but becomes more significant as the level of drive voltage E.sub.D and the beam current become large, as shown in FIG. 1. If the drive voltage E.sub.D has a low level and the beam current is small, the lack of sharpness of drive voltage E.sub.D in its rising and falling edges poses no problem. Therefore, it is neither necessary nor desirable that the scanning velocity of the beam be modulated uniformly for all levels of the drive voltage E.sub.D.
When the picture displayed on the screen becomes bright and the beam current becomes large, the size of the beam spot also becomes large with the result that the electron beam scans the adjacent color phosphor stripes simultaneously with the intended phosphor stripe and, hence, the degree of color saturation is lowered. However, such increase of the spot size of the electron beam can be compensated for by modulating the scanning velocity of the electron beam as set forth above. Nevertheless, when the beam current is small, the corresponding above increase of the degree of beam spot size and hence the lowering of the color saturation do not occur. Therefore, it does not benefit the quality of the displayed picture if the scanning velocity is uniformly modulated to increase the degree of color saturation when the beam current is small.